In some internal combustion engines, an air/fuel ratio sensor (for example, an oxygen concentration sensor) is arranged in an exhaust passage, and air/fuel ratio feedback control is performed by having the air/fuel ratio sensor detect exhaust gas components (for example, oxygen concentration).
In this case, a sensor element provided in the air/fuel ratio sensor is typically activated when heated to at least a predetermined temperature, thereby becoming capable of detecting the exhaust gas components. Accordingly, a sensor heater for heating the sensor element is often disposed in the air/fuel ratio sensor.
In an internal combustion engine having an air/fuel ratio sensor such as that described above, moisture generated during combustion is contained in the exhaust gas that is discharged from the internal combustion engine at the time of start-up, and in certain cases, this moisture forms water droplets that come into contact with the air/fuel ratio sensor. Further, condensed water is generated when exhaust gas remaining in the exhaust passage is cooled following a previous engine stoppage, and this condensed water may spatter onto the air/fuel ratio sensor when the internal combustion engine is started.
Hence, a part of the sensor element, which reaches a high temperature after being heated by the sensor heater, is cooled rapidly upon contact with the water droplets, leading to thermal shock, and as a result, damage such as breakage of the element may occur.
In actuality, however, control is often performed to restrict energization of the sensor heater until the temperature of the exhaust passage in the vicinity of the air/fuel ratio sensor has risen sufficiently. According to a known technique described in Japanese Patent Application Publication No. JP-A-2005-105960, for example, a set temperature for starting an operation of a heater is modified to a high side when an outside air temperature at the time of engine start-up is lower than an outside air temperature threshold, and the set temperature is set even higher when a temperature difference between an engine cooling water temperature and the outside air temperature at the time of engine start-up is larger than a temperature difference threshold.
Further, as described in Japanese Patent Application Publication No. JP-A-2004-353495, for example, a technique in which an increase rate of a throttle opening is restricted when it is determined that an air/fuel ratio sensor may be spattered by water has been proposed. In this technique, an exhaust gas velocity is not increased rapidly, and therefore a margin for evaporation can be provided before moisture in the exhaust passage spatters. Further, there is no limit on increments in an intake air amount, and therefore acceleration can be achieved through an acceleration operation. However, in these conventional techniques, the start of air/fuel ratio feedback control may be delayed, leading to an increase in emissions during start-up of the internal combustion engine.
Meanwhile, in another known technique, as described in Japanese Patent Application Publication No. JP-A-2002-227630, for example, the opening/closing timing of an exhaust valve is advanced during start-up of an internal combustion engine, thereby raising an internal cylinder pressure when an intake valve is opened, and as a result, compressed gas spit-back to the intake system is promoted, enabling atomization of the injected fuel and combustion stabilization during a cold start. In this case, a further effect is achieved in that gas which has reached a comparatively high temperature during or immediately after combustion in the internal combustion engine is discharged as exhaust gas, leading to an increase in the exhaust gas temperature. This effect may be employed to raise the temperature of the exhaust passage in the vicinity of the air/fuel ratio sensor early. In this conventional technique, however, control to modify the opening/closing timing of the exhaust valve to a timing corresponding to the operating condition is often begun by a variable valve timing mechanism when the cooling water temperature of the internal combustion engine reaches a predetermined value. Hence, the control for advancing the opening/closing timing of the exhaust valve is terminated early, making it difficult to increase the temperature of the exhaust passage efficiently.
An object of the present invention is to provide a technique for use in an internal combustion engine having an air/fuel ratio sensor that is disposed in an exhaust passage and can detect an air/fuel ratio in a heated state, with which the spattering of water droplets in the exhaust passage at the time of start-up can be terminated early, thereby advancing the heating timing of the air/fuel ratio sensor such that emissions during start-up of the internal combustion engine can be improved.